1. Field of the Invention
This invention relates to an anti-skid control system for motor vehicles of the type that the drive system thereof is switchable to a direct-coupled four-wheel drive system (referred to as direct-coupled 4WD system hereinafter). More particularly, the invention pertains to such an anti-skid control system which is adapted, during anti-skid control operation, to cause a pressure reduction starting point to occur earlier when the drive system of the motor vehicle is switched to the direct-coupled 4WD system than when the drive system is other than the direct-coupled 4WD system, thereby making relatively small the speed difference between a high peak and a low peak of the wheel speed, and thus making it possible to achieve smooth anti-skid control operation even when the direct-coupled 4WD system is working. The expression "direct-coupled 4WD system" used herein refers to the cases where no center differential gear is provided or the cases where even if a center differential gear is provided, the differential gear is locked so that at least the drive shafts of the front and rear wheels are coupled directly to each other.
2. Description of the Prior Art
There has heretofore been proposed an anti-skid control system for motor vehicles, wherein a reference wheel speed is set up which is made to follow the wheel speed to be reduced by brake pressure buildup, with such a speed difference between the two speeds that the reference wheel speed is lower by .DELTA.V than the wheel speed, the reference wheel speed which also is decelerated, from a point of time when the deceleration of the wheel speed reaches a predetermined level onward, with a constant deceleration gradient (-1 g) to hold the predetermined level of deceleration; and when the reference wheel speed becomes equal to the wheel speed, reduction of the brake pressure is started so that the wheel speed is thereby changed from deceleration to acceleration. An example of control modes which occur in such an anti-skid control system is shown in FIG. 4(A), wherein Vw indicates the wheel speed; Vt indicates the reference speed; t1, t2, . . . indicate points of time when reduction of the brake pressure is started; and 100 shows the manner in which the brake pressure is increased, reduced and held. As will be seen from FIG. 4(A), the wheel speed is caused to change with a relatively great speed difference between a high peak and a low peak thereof, so that a sufficient braking force can be secured.
When the drive system of the motor vehicle is switched to the direct-coupled 4WD system, however, if the wheel speed is so greatly changed, then a difference will be caused to occur between the average speed of the front axle and that of the rear axle, so that "jerky feeling" will tend to be produced in the anti-skid control operation.